There isn't a surefire way to get your player 48 minutes in 1 game. There's always a chance of foulout, injury or ejection. Only using 5 players in a game gives you the best chance, but if you want backups at other positions you always have risk of garbage time substitutions or low stamina that could play a factor.
If you really want to ensure the always 48min, make sure you have a less important trainee whose training you are willing to sacrifice for the other trainees.